Inorganic pigments are used as opacifiers and colorants in many industries including the coatings, plastics, and paper industries. In general, the effectiveness of the pigment in such applications depends on how evenly the pigment can be dispersed in a coating, in plastic or in paper. For this reason, pigments such as titanium dioxide are generally handled in the form of a finely divided powder. Titanium dioxide powders are inherently dusty, however, and frequently exhibit poor powder flow characteristics during the handling of the powder itself, especially during formulation, compounding, and manufacture of end-use products.
While free-flowing powders with low dust properties can be obtained through changes in the process for making these materials, these powders usually also exhibit reduced opacifying properties. To this end, chemical modification of titanium dioxide pigment surfaces has been the preferred approach to achieving the desired balance of pigment opacity and flow characteristics. Thus it is known in the art that the wetting and dispersing properties of titanium dioxide pigments can be improved by exposure to certain inorganic treatments, for example, depositing inorganic metal oxide and/or metal hydroxide coatings on the surface of the titanium dioxide. In addition, certain other chemical modifications of titanium dioxide pigment surfaces, involving treatment with organic compounds such as certain organic polyols, are also known to improve pigment performance, including helping to reduce the tendency of a pigment to adsorb moisture and improving the pigment's gloss characteristics, particularly in coatings. In plastics, as another example, improved pigment dispersion through inorganic and/or organic surface treatment of a titanium dioxide pigment likewise results in better processing and improved uniformity of color.
With regard to the present invention, it is, in particular, known to treat titanium dioxide pigment surfaces with organosilicon compounds. Organosilicon compound surface treatments serve to enhance the compatibility between the pigment and organic polymers in which the pigment is placed and so improve the reinforcing properties of the pigment when formulated into organic polymer matrices, such as in thermoplastic concentrates, but are especially of interest for their non-migrating character as they react with and are bound to the pigment surface.
The most advantageous organosilicon treatment in more general terms will depend on what particular end-use is intended for the surface-treated pigment, whether in thermoplastics, coatings or in paper. Accordingly, many patents have been issued disclosing methods for improving titanium dioxide pigments wherein an organosilicon compound is deposited onto the pigment surface prior to its incorporation into such end use materials as plastics, and to a lesser extent in coatings, inks and paper.
For example, U.S. Pat. No. 3,132,961 discloses a process for rendering finely divided non-alkaline filler material hydrophobic by contacting said finely divided filler material with a diorganopolysiloxane in the presence of an acid. Among the filler materials described are silica, clay, iron oxides, and titanium dioxide.
U.S. Pat. No. 3,227,675 describes the surface treatment of kaolin clay with up to 10% by weight based on clay of an organo-functional silane for the purpose of providing improved clay-reinforcing properties to thermosetting resins.
U.S. Pat. No. 3,567,680 discloses use of mercaptosilane grafted inorganic pigments in combination with aminosilane treated pigment, to achieve higher filler reinforcement of elastomers and plastics.
U.S. Pat. No. 3,834,924 discloses an improved process for producing surface modified, finely divided inorganic pigments comprising the addition of an amino organosilane to a high solids content aqueous dispersion of said inorganic pigment, with subsequent extrusion and drying. These pigments are useful as fillers for paper, paints, inks, and as reinforcing pigments in elastomers and polymeric materials.
U.S. Pat. No. 4,061,503 discloses the treatment of particulate titanium dioxide with a polyether substituted silicon compound which serves to enhance its employment in pigmented and/or filled paints and plastics, and in reinforced plastic composite compositions.
U.S. Pat. No. 4,141,751 discloses a process for the modification of hydrophilic particulate and/or fibrous crystalline and/or amorphous inorganic substances with silane coupling agents to produce materials which are eminently suitable for use as reinforcing filler/extender pigments in polymeric, polymeric alloy, and polymeric/ceramic alloy compositions.
U.S. Pat. No. 4,151,154 discloses compositions comprising inorganic particles containing on their surfaces a silane, its hydrolyzates or resulting condensates, which silane possesses at least two to about three hydrolyzable groups bonded to the silicon thereof and an organic group which contains a polyalkylene oxide group. These modified inorganic particles exhibit enhanced performance in pigmented and/or filled paints and plastics, and in reinforcing plastic composite structures.
U.S. Pat. No. 4,344,799 is directed toward titanium dioxide pigments with a coating of hydrophobicizing and hydrophilicizing organic substances, wherein the hydrophobicizing organic substance is an organic silicon compound and/or an organic phosphorus compound and the hydrophilicizing organic compound is an amino alcohol. These pigments are readily dispersible which simplifies their use in pigmenting laquers, plastics, and paper.
U.S. Pat. No. 4,375,989 discloses improved dispersibility of a titanium dioxide pigment by coating the pigment with an organic coating selected from the group comprising large-molecule fatty acids and their salts; organic silicon compounds, such as dimethyl polysiloxane; alcohols and polyalcohols. The titanium dioxide pigment also comprises a coating of an inorganic substance.
U.S. Pat. No. 4,514,231 is directed to the modification of natural oxidic or silicate fillers with water insoluble sulfur containing organosilicon compounds. The fillers are converted into an aqueous suspension and treated with the organosilicon compounds, in some cases in the presence of an emulsifier to improve reaction between the filler and the organosilicon compound. The compositions are particularly suitable for use in vulcanizable and moldable mixtures which are produced according to customary processes in the rubber industry.
U.S. Pat. No. 4,810,305 describes a modified hydrophobic colored or magnetic pigment or filler comprising a hydrophobic pigment or filler containing a surface treatment derived from an organopolyhydrosiloxane. Compositions include titanium dioxide pigments and are useful as pigments or fillers in synthetic resins.
U.S. Pat. No. 4,801,445 and U.S. Pat. No. 4,882,225 are directed toward cosmetics compositions containing modified powders or particulate materials having a silicone polymer film coated on substantially the entire surface, said silicone polymer being derived from at least one silicone compound containing an Si—H group.
U.S. Pat. No. 4,935,063 discloses inorganic fillers or pigments having simultaneous reinforcing effect and stabilizing effect on organic polymers obtained by a process comprising bringing the inorganic filler or pigment into contact with a solution, in an inert organic solvent, of a sterically hindered amine comprising one or more alkoxysilane groups in its molecule, maintaining the obtained mixture at higher than ambient temperature for a period of at least 0.5 hours, removing the solvent, and recovering the stabilizing filler or pigment obtained.
U.S. Pat. No. 5,035,748 describes an inorganic pigment comprising a content of at least 0.1% by weight and at most 5% by weight of one or more polyorganosiloxanes, wherein the polyorganosiloxanes have viscosities of 10 to 100,000 mPa·s and a relative molecular weight of 500 to 500,000, have no reactive or crosslinking groups, contain at least one C9-C25 Si-alkyl and/or one C9-C25 Si-aryl group per molecule, the total content of these groups in the polyorganosiloxane being 7-70% by weight and the other organic radicals contained in the polyorganosiloxane having 1 to 8 carbon atoms. These pigments can be used in laquers, emulsion paints, plastics, toners, magnetic recording materials, building materials, and enamels.
U.S. Pat. No. 5,501,732 discloses an improved process for preparing silanized titanium dioxide pigment for plastic and coating applications using a titanium dioxide slurry as a feedstock, wherein the viscosity of the high solids titanium dioxide slurry is reduced by adjusting the pH of the slurry in the range of about 7.5 to about 11.
U.S. Pat. No. 5,562,990 discloses organosilicon treatment of titanium dioxide particles coated with alumina or alumina-silica having a fluoride compound or fluoride ions associated with them to improve photostability and humidity resistance when incorporated in powder coatings and/or plastics.
U.S. Pat. No. 5,607,994, U.S. Pat. No. 5,631,310, U.S. Pat. No. 5,889,090, and U.S. Pat. No. 5,959,004 claim processes and compositions relating to white-pigmented polymers (particularly, polyolefins such as polyethylene) containing white pigments treated with at least one silane or a mixture of at least one silane and at least one polysiloxane resulting in improved processibility in thermoplastics compounding and improved performance properties, such as lacing resistance, in a polymeric matrix. Preferred silanes compounds are alkyl trialkoxysilanes.
U.S. Pat. No. 5,653,794 and U.S. Pat. No. 6,214,106 B1 disclose processes for the manufacture of hydrophobic inorganic oxides which comprise reacting inorganic oxide particles with organohalosilanes, preferably organochlorosilanes, to produce hydrophobic organosilane coated inorganic oxides, and compositions resulting from said processes. The inorganic oxide pigments prepared by these processes exhibit enhanced compatibility and adhesion between the pigment and organic matrices, such as thermoplastics. It is preferred that the organohalosilane compounds be reacted with the inorganic oxide particles in an aqueous slurry.
U.S. Pat. No. 5,707,437 describes titanium dioxide pigment particles with treatment of an organosilicon compound and boric acid and/or boron oxide. The resulting compositions, when incorporated into a polymer, exhibits humidity resistance, enhanced dispersion, and resistance to yellowing or discoloration of the pigmented polymer upon exposure to ultraviolet light. The resulting compositions are particularly useful in powder coatings and/or plastics applications.
U.S. Pat. No. 6,126,915 describes titanium dioxide powder with a greatly decreased volatile moisture content obtained by surface treating with a calcium salt and/or a silane coupling agent. Thermoplastic masterbatches containing this titanium dioxide powder do not exhibit defects due to foam generation resulting from high temperature processing.
U.S. Pat. No. 6,133,360 discloses thermoplastic resin compositions containing an aromatic polycarbonate resin and a surface modified titanium dioxide having a first coating and no further coatings. Preferred titanium dioxide first coating materials are polyols or polysiloxanes. The thermoplastic resin compositions exhibit improved resistance to streaking compared to such thermoplastic resin compositions which incorporate titanium dioxide having a first coating and at least one additional coating.
U.S. Pat. No. 6,395,858 discloses aminopropyl-functional siloxane oligomers, to the processes for preparing said oligomers, and to their use as reinforcing agents, surface modifying agents, or in coatings.
U.S. Pat. No. 6,455,158 B1 relates to the silanization or surface treatment of minerals with alkylsilanes and alkylsilane copolymers and to alkylsilane copolymers useful for surface treating pigments or fillers. The alkylsilane copolymers comprise at least two different monomers and find utility for the surface treatment of white pigments, such as titanium dioxide, for improving the dispersibility and processibility of the pigments when compounded with a polymeric material such as polyolefins.
U.S. Pat. No. 6,573,018 B2 describes surface-treated metallic oxide fine powders comprising a silane coupling agent containing primary amino groups useful in powder coatings or electrophotographic toners.
U.S. Pat. No. 6,576,052 B1 discloses titanium dioxide particles comprising a coating layer of an aluminum phosphate compound and a coating layer of the hydrolyzate of an organosilane compound, said pigment exhibiting improved light fastness, lacing resistance, and dispersibility in plastics.
U.S. Pat. No. 6,616,746 B2 describes a titanium dioxide pigment having on the surface a coating layer comprising a polyhydric alcohol and a hydrolysis product of an aminosilane compound. The pigment is useful as a coloring agent for plastics.
U.S. Pat. No. 6,620,234 B1 provides a treatment method for rendering titanium dioxide pigment hydrophobic by predispersing a suitably reactive organohalosilane into an aqueous media, using intensive mixing means to form a reactive dispersion, followed by exposing titanium dioxide particles to said reactive dispersion. The resulting pigments have good dispersibility in nonpolar substances such as plastics.
U.S. Pat. No. 6,663,851 B1 discloses a product obtained by treating surface-modified, pyrogenically produced titanium dioxide with at least one ammonium-functional silane, useful in the field of cosmetics in sunblocks, in toner powders, in paints and varnishes, in silicone rubber, and as abrasives and polishes.
U.S. Patent Application Publication No. US 2002/0172697 A1 describes a metal oxide-organopolysiloxane hybrid powder, a method for the preparation thereof, and a cosmetic composition containing said powder.
U.S. Patent Application Publication No. US 2003/0027896 A1 discloses a surface modified inorganic oxide powder having a surface modified with a mixed solution, which includes an organopolysiloxane and a silane compound. The resulting powders improve reinforcement of polar resins.
U.S. Patent Application Publication No. US 2003/0079655 A1 discloses a titanium dioxide pigment having a surface coating layer comprising a polyhydric alcohol and a hydrolysis product of an aminosilane compound, has a dispersibility of not more than 20 kg/cm2 as judged in terms of resin pressure increase, and is excellent in hydrophobicity and dispersibility. The pigment is useful as a coloring agent for plastics.
U.S. Pat. No. 6,770,327 discloses aminoalkylalkoxy silane mixtures comprising optionally, alkyl or hydroxyalkyl-functionalized siloxanes, to processes for preparing said mixtures, and to their use as reinforcing agents, surface modifying agents, or in coatings.
U.S. Pat. No. 6,841,197 discloses oligomer mixtures of n-propylethoxy silanes, to processes for preparing said mixtures, and to their use as reinforcing agents, surface modifying agents, or in coatings.
DE 197 51 857 A1 describes a method for producing phosphonatosiloxane-treated inorganic particles by incorporating organophosphonate compounds into organosiloxane compounds, such compounds being useful in plastics applications.
European Patent Specification EP 1 065 234 B1 relates to novel silicones for powder treatment, powders having the surface treated with such silicones, and cosmetic materials containing such surface-treated powders, wherein the surface treatment imparts to the powder a high affinity for fats and oils, including ester oils, glycerides, silicone oils, and fluorinated oils.
European Patent Specification EP 1 245 646 B1 describes titanium dioxide pigments having excellent light fastness and hydrophobic characteristics wherein the titanium dioxide is coated with an aluminum phosphate compound followed by a surface treatment with a hydrolyzate of an organosilane compound yielding a pigment particularly suited for use in plastics.
European Patent Specification EP 1 424 373 A2 relates to hydrophilized powders wherein the powder surface is treated with a polyether-modified silicone, and to their application in cosmetics, coatings, and inks.
U.S. Patent Application Publication No. US 2005/0129602 A1 discloses a process for production of titanium dioxide pigment and resin compositions comprising coating the hydrolysis product of an alkylsilane compound containing at least one C6H13 group by dry processing on surfaces of particles of titanium dioxide.
From the citations given above it is clear that many uses of organosilicon compound-treated pigments have been documented. However, in none of the references cited above are processes disclosed which describe aqueous treatment of pigments with alkyltrialkoxysilanes and dialkyldialkoxysilanes at pH values of between about 3.5 and about 7 in the presence of monoprotic acids. Alkyltrialkoxysilanes and dialkyldialkoxysilanes tend to oligomerize and crosslink to a lesser extent in this pH range and so are more effectively and easily applied to a titanium dioxide pigment as a surface treatment. Nevertheless, the slow rate of reaction of alkyltrialkoxysilanes and dialkyldialkoxysilanes in this pH range tends also to lead to undesired increases in manufacturing cycle-time, perhaps providing an explanation for the comparative absence of art pertaining to the surface treatment of titanium dioxide pigments with organosilicon compounds in this mildly acidic pH range.
Prior to treatment with organosilicon compounds, rutile titanium dioxide is commonly produced from titanium tetrachloride using vapor phase oxidation processes as disclosed in any number of patents and other printed publications, for example, in U.S. Pat. Nos. 3,208,866, 3,512,219, 5,840,112, 6,207,131 and 6,350,427. The reaction effluent from these vapor phase oxidation systems is generally cooled immediately upon leaving the reaction chamber, yielding a solid, agglomerated titanium dioxide intermediate.
This intermediate typically undergoes further processing steps in order to provide a finished product suitable for the uses listed above, including:
(1) dispersing the intermediate (or crude) material in an aqueous medium using a dispersing agent such as a polyphosphate,
(2) wet milling the resulting slurry to achieve a reduced particle size,
(3) precipitating inorganic oxides such as silica or alumina onto the particle surfaces of the wet milled titanium dioxide slurry,
(4) recovering the alumina- and/or silica-treated titanium dioxide pigment from the aqueous slurry by filtration,
(5) washing the filtered product to remove residual salts and impurities,
(6) drying the washed filtered product, and
(7) dry-milling the dried pigment using a fluid energy mill.
The deposition of inorganic oxides according to step (3), such as with silica or alumina onto the wet-milled titanium dioxide, is known to provide some desired pigment end-use properties as well as enabling the pigment to be recovered and washed using conventional vacuum-type and/or pressure-type filtration systems during manufacture.
For example, silica is typically added to impart improved resistance to the deleterious effects of ultraviolet light in pigmented end-use applications, whereas alumina is typically added to ensure smooth processing through filtration, drying, and fluid energy milling. Unfortunately, in some cases the presence of the inorganic oxides has also been observed to reduce the dispersibility of the dry pigment in coatings and thermoplastics so that as an alternative to added alumina, for instance, polymeric flocculants and/or multivalent metal ion flocculating salts have been added to the wet milled titanium dioxide dispersion in order to enable the pigment to still be collected and recovered using conventional vacuum-type and/or pressure-type filtration systems. However, the polymeric flocculants frequently themselves detract from the performance of the processed titanium dioxide product.
When spray drying is used to dry the titanium dioxide material following step (5), the washed titanium dioxide material is typically diluted with additional carrier liquid to enable delivery of the titanium dioxide material slurry to the spray dryer system as a fluid rather than a semi-solid. This dilution step, while enabling more practical and consistent conveyance of the semi-solid material to the spray dryer itself, can result in slower spray dryer throughput rates due to the presence of the additional carrier medium, as well as higher energy costs associated with the removal of the larger quantities of carrier liquid. One attempt to provide an improvement to this situation, wherein the washed titanium dioxide material, often referred to as “press cake” or “filter cake”, is converted into a low viscosity slurry via the addition of an alkalinizing agent, is described in commonly-assigned and co-pending U.S. patent application Ser. No. 10/928,387, filed Aug. 30, 2004 for “Improved Process for making Titanium Dioxide and Resulting Product”. In this particular case, the low viscosity slurry enables the efficient spray drying of titanium dioxide pigment intermediate while at the same time providing a pigment with improved processibility when formulated into thermoplastics. However, the usefulness of the disclosed viscosity reduction process is explicitly restricted to pigments wherein substantially no inorganic oxides have been deposited on the titanium dioxide material to be spray-dried.
Several references describe alternate methods for rendering pigment press cake or filter cake into a low viscosity fluid:
U.S. Pat. No. 4,186,028 describes improved fluid aqueous pigment dispersions, including titanium dioxide dispersions, comprising employment of a phosphonocarboxylic acid or salt thereof as a dispersion aid. In one particularly preferred embodiment, filter cakes which are normally difficult to transport are liquified by the addition of phosphonocarboxylic acids and transported in thin form to a drying or calcining unit to save energy costs.
U.S. Pat. No. 4,599,114 describes the treatment of titanium dioxide and other pigments with a surfactant compound consisting of the reaction product of a diamine, a carboxylic acid, and a fatty acid, to enhance the performance of the pigment in paints, plastics, paper making compositions, and reinforced plastic composite compositions. In one example, titanium dioxide press cake is converted into slurry form via vigorous mixing of the press cake with the inventive surfactant compound.
Lastly, U.S. Pat. No. 6,139,617 discloses an improved titanium dioxide pigment exhibiting improved gloss and dispersibility in surface coatings comprising titanium dioxide having deposited thereon a treating agent comprising at least one amine salt of a monoprotic acid.
Despite all the work and effort evidenced in the prior art relating to the development of improved organosilicon compound surface treatments for pigments, or relating to the development of improved processes for the manufacture of organosilicon compound-treated pigments, further improvements are continually being sought.